Structure Of The Crustal And Upper-mantle In Southeastern Tibetan And Its Dynamic Interpretation

Surface wave tomography and linear iterative inversion is the most effective tool for investigating the S-wave velocity structure of the crust and upper mantle.The structure,deformation mechanism and dynamics process of the crust and upper mantle in SE Tibet and Tengchong volcanic field(TCVF),the two hotspots in Chinese geosciences,are of significance for better understanding the evolution,uplift and materials escape of the Tibetan Plateau and the deep tectonic setting,magmatic activity in TCVF.Based on the two-step procedure,the azimuthal anisotropy model and the 3D S-wave velocity structure model of the region are constructed.The phase velocity and azimuthal anisotropy model of the SE Tibet are obtained from the Rayleigh wave anisotropy tomography.Subsequently,the 3-D S-wave velocity structure model of the crust and upper mantle in the SE margin of the Tibetan Plateau is constructed by linear iterative inversion with surface wave dispersion data.Based on the two models,the problem of regional correlation dynamics is discussed.The main contribution and conclusions of this dissertation are listed as follows:(1)We collect plenty of waveform data of the vertical components from 58 permanent stations and 74 portable stations.Based on interstation cross-correlation method and image analysis,11576 phase velocity dispersion curves are extracted.After quality control,3124 high-quality dispersion curves of 10-60s on the independent path are obtained.(2)The phase velocity and azimuthal anisotropy model of SE Tibet is obtained by the continuous Tarantola nonlinear inversion method,the model provides seismological evidence for the existence of asthenosphere upwelling beneath the Indo-China block and east-central Tibet and layer lithospheric structure of the Eastern Himalayan Syntaxis(EHS).In addition,by analyzing the anisotropic variations and dynamics features of the upper mantle,we observed that the deformation of the upper mantle in east-central Tibet is caused by the sustained subduction of the India lithospheric,and the asthenosphere upwelling has little effect on the deformation of the upper mantle.In the Indo-China block,the deformation of the upper mantle is mainly caused by the combined action of the LPO of the mantle peridotite minerals and the upwelling of the deeper mantle material.(3)Based on the anisotropic model,the deformation mechanism of the crust and upper mantle of SE Tibet is analyzed by the fusion of multi source data(e.g.,Pms/SKS splitting data,strain rate data and absolute plate motion rate),the crustal deformation is much more complex than that of the upper mantle.For the crust,the deformation of Songpan-Ganzi block may be related to the non-uniformity of the relative sliding of the block on both sides of the fault systems,which is caused by the east-west tectonic movement for plateau materials;the deformation of the Lhasa block and EHS are obviously affected by the subducting Indian lithosphere;the deformation of the Qiangtang block and Indo-China block is mainly affected by the strike-slip motion of the regional faults(e.g.,Nujiang fault,and Red river fault)and sutures(e.g.,Bangong-Nujiang suture and Jinsha river suture);the deformation of the Sichuan-Yunnan diamond block may be related to the southeastern extrusion of the plateau material;the deformation of the Yangtze block may be determined by the southeastern escape of the plateau materials and the stable lithosphere structure.In the upper mantle,the deformation of these blocks is mainly controlled by the absolute plate motion,except the east-central Tibet and Indo-China block affected by upwelling of asthenosphere.(4)The present study utilized a linear iterative inversion method to construct the 3-D S-wave velocity structure of the crust and upper mantle in the southeastern margin of the Tibetan Plateau(Yunnan region).The model reveals that there are two parallel low-velocity channels(marked as A and B)in the mid-lower crust,and this study interpret as channels of crustal flow.In conjunction with previous studies and our inversion results,we believe that the low-velocity channel A may be connected with the magma chamber of the crust in the TCVF,while the low-velocity channel B has a short branch north of Kunming toward the Mile-Shizong fault,and finally,it is connected with the low-velocity channel A on the southwest flank of the Red river fault.(5)A dynamics model of the volcanic magma migration is proposed based on the S-wave velocity structure in TCVF,revealing the upward migration pattern and deep dynamics process of the volcanic magmatic material.The formation of low-velocity anomalies in the crust is may be caused by two different migration patterns of magmatic materials.One migration pattern is upward migration by crack seepage,and the other is lateral migration from upper mantle magmatic material.There may be three possible causes for this low-velocity anomaly in the upper mantle.The first possibility is caused by the upwelling of hot mantle material from the deeper mantle;the second possibility is caused by the existence of residual magma,which has not cooled or solidified in the lithospheric fault systems;the third possibility is associated with the upward moving of the deep mantle-derived melts,which is caused by the ridge subduction along the 410 km discontinuity or the eastward subduction of the Indian plate under the Eurasian plate.In summary,the phase-velocity anisotropic model and the 3-D S-velocity structure model are proposed for better understanding the lateral variation of the crust and upper mantle in SE Tibet,and knowledging about the complex deformation mechanism,tectonic setting and deep magmatic dynamic process of the region.The effect of the plate subduction on crust-mantle deformation and volcanic magmatic activity is analyzed to promote the establishment of regional dynamic model,furthermore,it also provides deeper understanding and awareness for the evolution and uplift of the Tibetan Plateau.